Process for carburizing and carboni-triding iron and steel



PROCESS FOR CAR'BURIZING AND CARBONI- TRIDINGIRONANDS TE EL I JohannesMiiller, Frankfurt am MaihQGiiimahy, assignor to Deutsche GoldundSilber-Scheideanstalt vormals Roessler, Frankfurt (Main), Germany 1 NoDrawing. Application June 20, 1958 Serial No. 743,467

Claims priority, application; Germany June 26, 1957 8 Claims. (Cl.14815. 5)

The present invention relates to an improved process 7 for carburizingand carbonitriding iron-and steels in,

cyanide and cyanate containing salt baths whereby espeoially increasedcase depths can be obtained. It is known that iron and steel articlescan be cemented, that is, carburized to a certain depth below theirsurface by treatment in fused salt'baths containing cyanides. The saltbaths which are known for this purpose contain one or more cyanides asthe carbon source and substantial proportions of alkali metal and/oralkaline earth metal haldies. The decrease in carbon content withincreased distance from the surface of the articles and therefore alsothe time required for the carburization depends essentially upon thecyanide-content of the salt. bath, the presence or absence ofaccelerators or activators, such as, for example, certain alkaline earthmetal halides, and particularly the carburizing temperature employed.

Baths without an activator produce a low carbon content in the edge zonebut the nitrogen which is-simultaneously split'ofl from the cyanideproduces a high'surface hardness. However, deep case depths cannot beachieved with such baths and the cases cannot be ground off withoutcausing the hardness to decrease strongly. On the other hand, such bathsare suited for the production of very thin carburization cases attemperatures below the pearlite point, above 600 C. and preferably about650-700" C. The cases thus obtained are very hard and in view of the lowtemperatures employed the distortions produced by such treatment areonly very slight. It was, however, previously not possible to increasethe case depth obtained sufficiently so that this technique is onlyseldom used.

Baths without an activator in general contain more than 40% of cyanide.Because of high cyanide losses at high temperatures such baths are onlyoperated at temperatures below 900 C. In salt baths which containadditions which promote the liberation of carbon, the cyanide content isconsiderably lower and usually is about 10% and barium chloride in aquantity of about 50 to 55% is present as the main constituent of thebath. The usual working temperature of such baths is about 930 C. andthe use of substantially higher temperatures which would accelerate thecarburization is generally not possible, as otherwise impractically highcyanide losses are engendered. I

It has also been proposed to carburize iron and steels with cyanidecontaining baths predominantly composed of 2, As in general cyanidecontaining baths a small quantity of cyanate is automatically produced,an oxygen content in the baths is automatically provided so thatfit isonly necessary to add at least'one of the other elements; tellurium orselenium to such baths. In general,.0.05 .to

5% of such added element or elements snflicesand pref erably a 'quantitybetween 0.2% and 3% is employed:

According to the process of the invention the bathscontain 5 to 40% KCNor equivalent amounts of eg NaCNO, LiCNO, RbCNO and the like as well as0.1' to 60% NaCN or equivalent amounts of e.g. KCN;-

The remainder consists essentially of carbonates and halogenides espe-'cially chlorides and fluorides of alkaline metals and LiCN, RbCN, Ca(CN)and the like.

eventually of earth alkaline metals. Small quantities-of oxides arenearly always present. Mixtures of Na/KCO Also ood yields are" obtainedwith fluorides. The fused salt 'bathtemperatures and Na/KCl areverysuitable;

employed according to the invention lie between 600 and 1100C.,'preferably-between 650 and (3. Expediently, temperatures between 650and 930 C. are

used. Y

The following examples will serve to illustrate the unexpectedlyimproved results obtained with the process according to the invention.

. Example 1 To a bath of 47% ture of 700 C. for 2 hours. Thereafterthethickness of the so-called transition zones and .the total thicknessof the case obtained on the treated samples were deter-' minedmicroscopically. The total thickness was 0.14

mm. and the thickness of the transition zone was 0.041

Example 3 To a bath of 50% NaCN, 15% KCNO and the remainder potassiumcarbonate, 0.7% sodium telluride are added. Samples of a low carbonsteel (containing 0.15 of carbon) were treated in this bath at atemperature of 700 C. for 2 hours. Thereafter the thickness of theso-called transition zones and the total thickness of the case obtainedon the treated samples were determined microscopically. The totalthickness was 0.17 mm. and the thickness of the transition zone was 0.02mm.

Example 4 A bath of 50% sodium cyanide, 10% sodium cyanate and theremainder sodium carbonate was compared with a bath of the samecomposition having 1% of selenium added thereto in the form of sodiumselenite. Samples of a low carbon steel (containing 0.15% of carbon)were treated in such baths at a temperature of 700 C.

' for 1, 2, 5 and 10 hours. Thereafter the thickness of the 7 additionoxygen at least contain one of the elements selenium and tellurium.

so-called transition zones and the total thickness of the case obtainedon the treated samples were determined microscopically. The followingtable compares the thicknesses attained.

p 7 Patented Mar. 8, 1960,,

NaCN, 35% KCNO and the re mainder K/NaCO 0.5% sodium selenide are added.Samples of nearly carbon free steel (Armco-iron with; 0.05% carbon) weretreated in this bath at a 'tempera-- Without Se With Se Hours TotalTrans. Total Trans. Thickness, Zone, Thickness, Zone,

mm. mm. mm. mm.

The, portion of the case not forming a part of the transition zone wasessentially martensitic. seen that,,even only after 1 hours treatment,the total thickness of the case was substantially greater when the Vselenium containing hath was employed. The thickness of; the transitionzone is even approximately twice that when thebath. in which theyselenium was omitted was employed. The ratio becomes worse with longertreating periods but is noticeably still to the detriment of the bathnotcontaining selenium. It is particularly remarle.

It will be.

the group consisting of selenium and tellurium has been added, said saltbath being maintained at a temperature between 600 and 1100 C;

2. The process of claim 1 in which the temperature of said salt bath ismaintained between 650 and 930 C. 3. The process of claim l in'whichsaid cyanides and cyanatesare alkali metal cyanides and alkali metalcyanates.

4. The process lof claim 1 in which said cyanides and cyanates arealkalineearth metal cyanides and alkaline earth metal cyanates.

5. The process of claim 1 in which said added element is selenium.

6. The process of claim 1 inwhich said added element is tellurium. 7 V

7.' A process for carburizing and carbonitriding iron and steel whichcomprises treating such metals in a fused 7 salt bath containing alkalimetal cyanides equivalent to able, however, that in selenium free bathsthe sum of thicknesses of the martensitic zone and the'transition 7,zonedoes not increase upon increase of the, treating period from 1 to10 hours, whereas in selenium containing baths the thickness increasesover five fold.

Analogousresults were obtained with tests carried out at; highertemperatures. Also, replacement of the selenium by tellurium provides anincrease of the same order in.

ofv NaCN as well as cyanates equivalentto. 5-40% of' KCNO and theremainder essentially consisting of carbonates and halides of alkali andalkaline earth metals, to which 0.05-5% of at least one element selectedfrom 01-60% of NaCN as well as alkali metal cyanates equivalent to 5-40%KCNO and the remainder essentially consisting of carbonates and halidesof alkali and alkalineearth metals, to which 0.05-5% of selenium hasbeenadded, saidsalt'bathbeing maintained at a temperature between 600and 1100 C.

8..A process for carburizing and carbonitriding iron and steel whichcomprises treating'such metals in a'fused salt bath containing. alkalimetal cyanides equivalent to.-

7 01-60% of NaCN-aswell as. alkali metal cyanates equivalent to 5-40%KCNO andthe remainder essentially con-.- sistingrof carbonates andhalides of'alkali and alkaline: earth metals, to which 0.055% oftellurium has been: added; said salt bath beingmaintained at atemperature:

between 600. and 1100 C.

References Cited in the tile of this patent Troup: Materials andMethods, vol. 44, September 1956, pages 110-113 (TA 401 M5).

Johnson: Metal Working andHeat Treatment Manual, v0l.-III, pages -64,page 55 relied upon (TS 205 16- V3).

Materials and Methods, vol. 25, March 1947, page 108 (TA 401 M5).

1. A PROCESS FOR CARBURIZING AND CARBONITRIDING IRON AND STEEL WHICHCOMPRISES TREATING SUCH METALS IN A FUSED SALT BATH CONTAINING CYANIDESEQUIVALENT TO 0.1-60% OF NACN AS WELL AS CYANATES EQUIVALENT CONSISTINGOF CARKCNO AND THE REMAINDER ESSENTIALLY CONSISTING OF CARBONATES ANDHALIDES OF ALKALI AND ALKALINE EARTH METALS, TO WHICH 0.05-5% OF ATLEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF SELENIUM ANDTELLURIUM HAS BEEN ADDED, SAID SALT BATH BEING MAINTAINED AT ATEMPERATURE BETWEEN 600* AND 1100*C.